Method and apparatus for distributing a globally accurate knowledge of time and frequency to a plurality of a high definition television studios

ABSTRACT

A method and apparatus for distributing time and frequency information to a plurality of studios such that the studios can then use the time and frequency information frequency and timelock their studio components to the global reference. The apparatus includes various embodiments for facilitating the distribution of time and frequency depending upon the type of digital network that is used for distributing the television signals.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 60/077,717, filed Mar. 12, 1998, which is hereby incorporatedby reference.

The invention relates to systems that distribute time information tomultiple remote locations and, more particularly, the invention relatesto a method and apparatus for distributing a time and frequencyinformation to multiple remotely located high definition televisionstudios.

BACKGROUND OF THE DISCLOSURE

A modern television studio contains a plurality of studio componentsthat must be accurately time synchronized to facilitate switchingbetween television signals produced by various sources within thestudio. In a conventional NTSC system, the timing reference for bothanalog and digital signals within the studio are based on the notion of“system genlocks”, in which all studio components are locked to a studioreference via a dedicated timing reference cable. As such, each andevery component of an NTSC studio has a signaling cable as well as agenlock timing reference cable connected to it. Within a genlock system,frames of video information are moved synchronously between components.The genlock signal is instrumental in ensuring that framesynchronization is maintained between all the components of a studio.However, in a situation where data is passed between components usingcompressed digital formats, such as is used in high definitiontelevision (HDTV) broadcast studio, the genlock system is ineffective atmaintaining synchronization between components with respect to thecompressed digital signals.

In a HDTV studio, the various components within the studio arephysically connected to each other through a central switch called astudio data router. This router is responsible for processing not onlythe video bitstreams that are coupled to the studio, but also to providecommand and control services to the studio and any other services thatare required by the studio. The low-level transport for data within thestudio is asynchronous transfer mode (ATM) which is well suited forproviding multiple data streams to a single point. Heretofore, there hasnot been developed a timing and frequency distribution system that wouldbe equivalent to the system genlock of an NTSC studio for a HDTV studio.Since many studios share data and information through satellitecommunications and cable communication systems, it would be advantageousto have a global sense of time to be distributed not only to componentswithin the studio, but also to all studios within a network such thatretiming of signals at any studio location is not necessary prior tobroadcast.

Therefore, a need exists in the art for a method and apparatus fordistributing a global sense of time (and frequency) to a plurality ofstudios such that the studios can naturally time lock all of the studiocomponents to the global time reference.

SUMMARY OF THE INVENTION

The disadvantages associated with the prior art are overcome by a methodand apparatus for distributing a reference frequency and a referencetime to a plurality of high definition television (HDTV) studios suchthat the studios can then use the reference frequency and time to locktheir studio components to a global frequency and time reference. Theinvention includes various embodiments for facilitating the distributionof time and frequency depending upon the type of digital network that isused for distributing the television signals within a studio componentswithin a studio. The first embodiment is useful in a system that uses asynchronous optical network (SONET) for distributing television signalsto a plurality of studio components within a studio. This embodimentextracts a reference frequency signal from the SONET signals such thatall the components in the studio that are equipped with this embodimentcan be locked to the SONET frequency. Within this form of signaldistribution, the SONET path and line overhead bytes can be used todistribute a global sense of time of day information to the studiocomponents.

Another embodiment of the invention is a second method and apparatus fordistributing a global sense of time of day information to multiplecomponents within a studio. In this embodiment, the program clockreference within an MPEG transport stream is used for the distributionof time of day information to various studios and their studiocomponents. Since MPEG transport streams are used in all forms ofdigital television information distribution, this embodiment is usefulin SONET and non-SONET based distribution systems.

The last embodiment of the invention uses the internet engineering taskforce and network time protocol for distribution of time of dayinformation to the studios and their respective components.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a digital television distribution system based upon aSONET network;

FIG. 2 depicts an illustration of a HDTV television studio;

FIG. 3 depicts a block diagram of apparatus for extracting a referencefrequency from a SONET signal for a studio component;

FIG. 4 depicts a block diagram of an apparatus for extracting a time ofday signal from a SONET signal for a studio component;

FIG. 5 depicts a SONET STS-1 frame;

FIG. 6 depicts a SONET STS-1 synchronous payload envelope;

FIG. 7 depicts a block diagram of an apparatus for extracting time ofday information from an MPEG compliant bitstream; and

FIG. 8 depicts a block diagram for extracting time of day informationfrom the internet engineering task force network time protocol.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

FIG. 1 depicts a block diagram of a digital television signaldistribution system 100 that distributes digital television signalsamongst a plurality of digital television studios 116 ₀, 116 ₁, 116 ₂, .. . , 116 _(n) (hereinafter referred to collectively as 116). A portion101 of one of the studios 116 ₀ is depicted to illustrate one techniquefor distributing a global sense of time amongst a plurality of studios.

The digital television signal processed by a digital television studio116 generally complies with the moving pictures expert group (MPEG)specification (ISO/IEC 13818). The signal is supplied a bitstream source102 from a high definition television (HDTV) or digital television (DTV)studio 116. The bitstream source 102 generally takes the digitaltelevision signal from a plurality of remote sources such as from asatellite television network, cable television network or local sourcessuch as a tape drive or DVD player, or a camera producing a digitalbitstream directly from the studio or sporting event and the like. Thesignal or signals are processed within the studio and then may betransmitted to other studios, i.e., a digital network feed.

The bitstream source 102 processes those source streams to produce oneor more MPEG compliant bitstreams that are coupled to a PCR stamper 108.The program clock reference (PCR) field within each data packet in anMPEG compliant bitstream is a 42 bit field composed of a program clockreference base of 33 bits and a program clock reference extension of 9bits. The program clock reference base is in units of {fraction(1/300)}th of a system clock cycle, i.e., {fraction (1/90,000)} of asecond. Thus, the PCR is able to run for 2³³×{fraction (1/90,000)} of asecond, approximately 1.1 days before wrap around occurs.

PCR stamper 108 is coupled to a stable frequency reference 109 such as aglobal positioning system (GPS) receiver 106, SONET interface 112 orsome other frequency reference 118. The OR gate 120 enables any one ofthese frequency sources to be coupled to the PCR stamper 108. PCRstamper 108 is also coupled to a stable time of day reference 111 suchas a GPS receiver 106, Network Time Protocol Server 122 (discussed belowwith reference to FIG. 8) or some other time of day reference 124. Ofcourse, an actual implementation of a studio may contain only onefrequency and time of day reference. For example, to restamp a globalsense of time into the PCRs of an MPEG bitstream, the GPS receiver 106receives a global time reference through its antenna from the well-knownsatellite based GPS system. The time reference information (GMT time ofday) from the GPS receiver is coupled through the OR gate 120 to the PCRstamper 108. The PCR stamper 108 restamps the PCR fields within each ofthe MPEG compliant bitstreams such that all the bitstreams contain thetime of day generated by the GPS receiver (or any other time of dayreference). As such, all of the restamped bitstreams carry thesynchronous time of day signals. The bitstreams having the restamped PCRfields are coupled to a transport stream generator 110 which organizes(multiplexes) the various MPEG compliant bitstreams into a transportstream that is also MPEG compliant. The transport stream is then coupledfrom the transport stream generator 110 to a synchronous optical network(SONET) interface 112. The SONET interface couples the transport streamto a SONET network such that the stream is distributed to one or moreHDTV studios 116 ₁, 116 ₂, 116 ₃, . . . 116 _(n) (collectively 116) thatwill in turn broadcast the television program or programs that arecarried by the transport stream. The SONET interface generates the SONETsignals using a frequency reference of N×51.84 MHz such that all SONETsignals within the SONET network carry the same frequency reference. Forexample, the OC-12 signal is 12×51.84 MHz.

The present invention ensures that the studios 116 receive a commonfrequency reference as well as common time of day information such thatall of the studios 116 are locked to the same time of day informationand frequency reference to facilitate processing and switching betweenMPEG streams without concern for local synchronization of the signals.Consequently, if each studio restamps the PCR fields of any programmingthat is being transmitted to another studio in the manner describedabove (i.e., using a global sense of time such as that provided by GPS),then all the studios will synchronously process the information.

FIG. 2 depicts a block diagram of an illustrative digital televisionstudio 116. The studio has at its core, a studio data router 200 thatcouples data from any one component of the studio to any other componentwithin the studio. The studio components 204 include encoders, programservers for supplying programming from a digital disk array,transcoders, video format converters, productions switches, auxiliarydata sources, a high definition television transmitter, video tapesources and recorders, digital television monitors and the like. Withinthese components is also a network interface device 202 that couples thestudio data router 200 to the SONET wide area network 114. Both thestudio router 200 and the SONET wide area network 114 receive a commonclock. Although SONET is illustratively used to distribute the digitaltelevision signals across a cable network, any other cable network thatuse a common frequency reference and that is capable of carrying MPEGtransport streams would also suffice. The router 200 extracts time ofday and frequency reference information from the network signals. Thetime of day and frequency reference information is then provided to theremaining studio components 204. Alternatively, each studio componentmay have circuitry that extracts the time of day and frequency referenceinformation. In that instance, the router couples the time of dayinformation and frequency information to the components withoutperforming any extraction within the router. The apparatus forextracting this information either locally within the components orwithin the router is discussed below in reference to FIGS. 3-8.

More specifically, a studio router 200 performs stream splicing andswitching such that various sources of digital television signals can becombined and spliced together for broadcast. Consequently, the splicingand switching processes require that the digital information besynchronously provided to the router 200 such that splicing will notproduce any visible artifacts when the video stream is decoded within aviewer's television set. To ensure this synchronicity, a component 204(any one of the studio components locally or the router itself) extractstime of day information and frequency information from the networksignal such that all of the components within the studio can be lockedto the same time of day and frequency information. Similarly all otherstudios (116 in FIG. 1) connected to the network are also locked to thesame frequency and time of day such that any information coupled betweenthe studios is also locked to the same frequency and time of day source.

FIG. 3 depicts a block diagram of reference frequency generationcircuitry 308 (reference clock generator) that is a portion of a studiocomponent 204 that required a frequency reference. The frequencygenerator 308 comprises a SONET interface 300, a clock extractor 302, aphase lock loop (PLL) 304 and a local oscillator 306. In lieu of (or inaddition to) a SONET-based reference, the circuitry 308 also has inputports that couple to other frequency reference sources such as GPS 106or some other source 118. All the sources 300, 106, or 118 are coupledthrough an OR gate 310 to the clock extractor 302. The SONET interface300 directly attaches to and receives input from a SONET-based router(200 in FIG. 2). The clock extractor 302 extracts the frequencyinformation from the SONET signal. All other information (e.g., digitaldata) carried by SONET would be processed by the studio component and isoutput from the interface 300 on path 312. The free running SONET clockis defined as n×51.84 MHz+/−20 ppm (where n is an integer value). Fordelivery of 300 Mbps (mezzanine) compressed MPEG bitstream, an OC-12interface of 622.08 MHz would be used. From the 622.08 MHz clock signal,a phase lock loop 304 uses the SONET clock signal as a reference signaland locks the local oscillator (27 MHz) to the reference signal. The 27MHz local oscillator is based on the MPEG system level reference clockas described in the MPEG-2 specification. The system clock is requiredto be 27 MHz+/−810 Hz at the rate of frequency change the time of nomore than 75×10⁻³ Hz per second. The system clock (27 MHz) is related tothe SONET clock (51.84 MHz) in a ratio of system clock times 25 dividedby 48. The 27 MHz reference system clock is then distributed to thestudio component that contains the circuitry 308.

In accordance with the present invention, the first embodiment to conveya global sense of time of day requires each studio component have a GPSreceiver 106 to provide TOD and optionally its own reference clockgenerator 308 that is coupled to a commonly distributed 51.84 Mhz SONETclock reference, or some other frequency reference 118 through whichmultiple studio components can be frequency locked to one another. Assuch, each component generates its own local reference signal that mayor may not be the 27 MHz system clock. As such, the PLL 304 has an“other” output that represents that the PLL 304 may be locked to adifferent local oscillator or may derive multiple frequencies for agiven component where each of these frequencies is locked to the SONETclock signal.

FIG. 4 depicts a block diagram of a second embodiment of the presentinvention that generates and distributes a global sense of time of daywithin a studio using either the path or line overhead bytes within theSONET signal structure. SONET provides a digital multiplex format forthe wide bandwidth of optical fiber cable. The SONET hierarchy providesa basic signal of 51.840 Mbps per second and a byte interleavedmultiplex scheme that results in a family of digital rates and formatsdefined as a rate of N times 51.840 Mbps per second. The basic signalhas a portion of its capacity dedicated to overhead, and the remainingportion carries payload. The basic modular signal within SONET is calledSTS-1 with 51.840 Mbps per second data rate. The STS-1 opticalcounterpart is called OC-1 (OC is optical carrier) and its rate isdirectly converted from STS-1 after frame synchronous scrambling.

The STS-1 frame structure is depicted in FIG. 5. The frame structure 500contains 90 columns and 9 rows of 8 bit bytes for a total of 810 bytes.With a frame length of 125 microseconds (derived from 8000frames/second), STS-1 has a bit rate of 51.84 Mbps. The first threecolumns 502 are the transport overhead information that containsoverhead bytes of sections and line layers. Of the 27 bytes in thetransport overhead, 9 bytes are used for section overhead 506 (usedbetween line terminating equipment to section terminating equipment andbetween section terminating equipment) and 18 bytes are used for lineoverhead 508 (used between line terminating equipment). The remainingbytes within the STS-1 frame form the payload capacity 504. Within theSTS-1 payload capacity 504 floats a synchronous payload envelope (SPE)600, depicted in FIG. 6. The start location of the SPE is pointed to bypointer fields in the line overhead (fields H1 and H2) and may span twoSTS-1 frames. The SPE 600 contains a column of overhead called the pathoverhead 602 (used between path terminating equipment). Within thesection, line and path overhead bytes is a provision within the SONETprotocol to pass user data. Specifically the following fields areavailable for use with user data:

E1 Section Orderwire F1 Section User D1-D3 Section Data Com D4-D12 LineData Com E2 Line Orderwire F2 User Channel

The present invention uses these SONET user data fields for accurateconveyance of time of day to studio components. FIG. 4 depicts a blockdiagram of components that are used for extracting time of dayinformation from the user data fields of a SONET signal carried by thepath or line overhead bytes. The embodiment of the invention depicted inFIG. 4 forms a portion of a network interface device within a studiocomponent that requires synchronous time of day information. The device202 comprises a SONET interface 300 for interfacing with the SONETnetwork and extracting information on the SONET network that isaddressed to a particular studio component. The path or line overheadbytes extractor 400 removes user data information from the transportoverhead portion of the STS-1 frame structure. The user data containsencoded time of day information. The time of day (TOD) computer 402converts the user data into a local time of day for use by the studio.As such, the studio component receives a local time of day that issynchronized to a global time of day. The user data can include the timeof day in an encoded format so that it does not require a significantnumber of bits to encode and transmit.

Recognizing that the time of day services are provided by the router 200at FIG. 2, and that the router is directly connected to all studiocomponents (possibly with a repeater between those components), it isexpected that the most appropriate user data are those at the lineoverhead layer, i.e., either Line Data Com or Line Orderwire.

FIG. 7 depicts a block diagram of a third embodiment of the inventionwherein the program clock reference (PCR) of an MPEG compliant transportstream is used to transmit a global sense of time to various studiocomponents. This technique does not rely upon any of the SONET protocolfeatures such that any information distribution network can be used fordistributing the MPEG compliant transport streams within the studio. Solong as an MPEG compliant bitstream carrying a restamped PCR isavailable, the invention extracts the time of day information from thePCR field. The invention is incorporated into a network interface deviceof any studio component 204 requiring synchronous time of dayinformation. The value of the PCR is assumed to have been exactly zeroat 00:00.0 GMT on Jan. 1, 1970 and have been incremented continuallysince that time. Thus, every MPEG compliant bitstream containsperiodically distributed PCR fields within the data packets to maintainTOD and timing synchronization of the decoded video and audio signals.The invention contains an MPEG stream selector 700, a PCR extractor 702,and a TOD computer 704. Assuming that all of the PCR fields have beenrestamped with a GPS time of day signal in a manner such as thatdiscussed with reference to FIG. 1, the MPEG stream selector 700 selectsthe appropriate stream within the transport stream structure for use bythe studio components. The selected stream is further processed by thePCR extractor 702 to extract the program clock reference fieldinformation. The PCR field is a 42 bit field composed of a program clockreference base of 33 bits and a program clock reference extension of 9bits. The extractor 702 couples the PCR field data to the TOD computer704 wherein a time of day is computed from the PCR field data.

Video events, such as switching from one stream to another, occur onboundaries that are expressible as values of the system time. Everyswitchable unit (splice point or segment) has a duration that is amultiple of 30 Hz, 29.97 Hz or 24 Hz. For example, assume that at thestart of some sequence the system clock is 0 and that two short clipsare to be played one after the other. The first clip is a movie clip at24 Hz that contains 240 frames or 270,000,000 ticks of the system clock.At its conclusion the system clock would be at 270,000,000. If this werefollowed by a 90 frame clip of 29.97 Hz video that is 81,810,00 ticks ofthe system clock, the system time at the end of both sequences will haveadvanced 351,810,00. In fact, the granularity of all video segments andconcatenated video segments is an integral number of thousands of systemclocks ticks.

Whenever start and stop times are specified in the studio which do notcorrespond to an exact frame boundary, the next frame boundary after thespecified time is used, e.g., if the intention were to start the eveningnews at 6:00 PM Tuesday, and the translation of 6:00 PM to system clockcount came out to be 123,456,789,012 and if the previous stream was a 30Hz stream with splice points every 900,000 ticks, then the first slicepoint past that time must be between 123,456,790,000 and123,547,000,000. The exact value would depend on the PCR value thatcorresponds to frame boundary for the 30 Hz stream where a splice pointis located.

The invention distributes a global sense of time to ensure that splicingand switching from stream to stream occurs exactly at the appropriatetime from studio component to studio component. Such global timedistribution is instrumental in creating a system wide synchronizedtiming arrangement.

The fourth and last embodiment of the invention for distributing timewithin a HDTV studio obtains the time of day by using the InternetEngineering Task Force (IETF) Network Time Protocol (NTP) protocol asshown in FIG. 8. This protocol makes use of NTP client protocol entities(NTP client 804) running on the participating studio components whichexchange time of day messages with an NTP server 122 to adjust theirinternal clocks to maintain an accurate measurement of time of day. Assuch, a local sense of time, such as provided by a GPS receiver 106, canbe formatted by the NTP server 122 and routed to the studio componentsNTP through an NTP network router 806. The router 806 may distribute theNTP signals via any form of network, SONET, ETHERNET or other type.Details of the NTP protocol may be found in IETF Request for Comments(RFC) 1305, Mills, D., “Network Time Protocol (Version 3) SpecificationImplementation and Analysis”, RFC 1305, 1992.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. Apparatus for distributing a global sense of timeto a plurality of remote HDTV studios comprising: a time of dayinformation processor for inserting time of day information into adigital signal that is transmitted to a plurality of HDTV studios; atime of day extractor for extracting the time of day information fromthe digital signal; and a time of day distributor for distributing thetime of day to at least one component within each HDTV studio.
 2. Theapparatus of claim 1 wherein said time of day information processor is aPCR restamper that encodes time of day information in a PCR field of anMPEG compliant bitstream.
 3. The apparatus of claim 2 wherein the timeof day extractor comprises: an MPEG stream selector; a PCR fieldinformation extractor; and a time of day computer.
 4. The apparatus ofclaim 1 wherein said signal is distributed to the HDTV studios usingSONET and said time of day information processor comprises: means forinserting time of day information into an overhead bytes of a SONETsignal.
 5. The apparatus of claim 1 wherein said time of day extractorfurther comprises: a SONET interface; an overhead byte extractor; a timeof day computer.
 6. The apparatus of claim 1 wherein said overhead bytesare either the path overhead byte or the line overhead byte.
 7. Theapparatus of claim 4 further comprising a frequency extractor forextracting frequency information from a SONET signal.
 8. The apparatusof claim 7 wherein said frequency extractor comprises: a SONETinterface; a clock extractor; and a phase lock loop for locking an HDTVstudio clock to a SONET clock reference.
 9. The apparatus of claim 1wherein said time of day distributor is a data router.
 10. The apparatusof claim 1 wherein said time of day information processor is a NetworkTime Protocol server and said time of day extractor is an Network TimeProtocol client.
 11. A method for distributing a global sense of time toa plurality of remote HDTV studios comprising the steps of: insertingtime of day information into a signal that is transmitted to a pluralityof HDTV studios; extracting the time of day information from the signal;and distributing the time of day to a plurality of components withineach HDTV studio.
 12. The method of claim 11 wherein said inserting stepencodes time of day information in a PCR field of an MPEG compliantbitstream.
 13. The method of claim 12 wherein said extracting stepcomprises the steps of: selecting an MPEG stream; extracting the PCRfield information; and computing from the PCR field information a timeof day.
 14. The method of claim 11 wherein said signal is distributed tothe HDTV studios using SONET and said inserting step comprises the stepof: inserting time of day information into an overhead bytes of a SONETsignal.
 15. The method of claim 11 wherein said time of day extractingstep further comprises the steps of: extracting an overhead byte fromthe SONET signal; computing a time of day from the overhead byte. 16.The method of claim 11 wherein said overhead bytes are either the pathoverhead byte or the line overhead byte.
 17. The method of claim 14further comprising the step of extracting frequency information from aSONET signal.
 18. The method of claim 17 wherein said frequencyextracting step comprises the steps of: extracting a clock signal; phaselocking an HDTV studio clock to a SONET clock reference.
 19. The methodof claim 11 wherein said inserting step encodes time of day informationusing the Network Time Protocol.
 20. The method of claim 11 wherein saidinserting step encodes time of day information using a GPS signal.
 21. Amethod of distributing a reference frequency to a plurality of studiocomponents in a plurality of HDTV studios an HDTV studio comprising thesteps of: extracting frequency reference; and phase locking an HDTVstudio component clock signal to said frequency reference in each ofsaid plurality of HDTV studios.
 22. The method of claim 21 wherein saidfrequency reference is carried by a SONET clock reference signal. 23.The method of claim 21 wherein said frequency reference is carried by aGPS signal.